Thermoplastic foams made from styrenic polymers, such as polystyrene, have found extensive use, particularly as insulating materials. Genrally, insulating styrenic foams are produced in thicknesses greater than about one-half inch. The insulating value of such foams is measured in terms of heat conduction resistance or R-value, per one inch of foam thickness, and adequate insulating foams typically have R-values of about 5.0 per inch or greater. Styrenic insulating foams to meet government construction standards generally must also be dimensionally stable, i.e., they must have a maximum change in any of length, width or thickness of less than about 2.0 percent when subjected to a 158.degree. F. temperature for 24 hours.
These styrenic foams typically are made by mixing a volatile blowing agent with the styrenic resin under a controlled temperature and pressure sufficient to liquefy and plasticize the resin and to maintain the resin in an unfoamed state, and then extruding the resin-blowing agent mixture through a die into a zone of lower temperature and pressure which results in the formation of the styrenic foam. Blowing agent compositions for styrenic foam production generally are required first to have a system vapor pressure at the melt temperature of the styrenic resin sufficient to produce acceptable insulating foam, and second to provide sufficient plasticization to the styrenic resin to permit extrusion at suitable production rates. The blowing agent should also not have too great of a plasticizing effect on the styrenic resin, to avoid dimensional instability of the foam.
Dimensionally stable, insulating styrenic foams which retain an R-value above 5.0 per inch upon aging, are being produced commercially with blowing agents based on 1-chloro-1,1-difluoroethane (also known as "FC-142b", which will be used for convenience hereafter). FC-142b-based blowing agents eliminate the use of the chlorofluorocarbons, such as Freon 12, which are suspected of reacting with ozone after their release into the earth's atmosphere.
These hydrofluorocarbon blowing agents of reduced or zero ozone reactivity potential have sufficient plasticizing effect when used in conjunction with ethylchloride to achieve suitable production rates of insulating styrenic foams. Emissions of ethylchloride during foam production are considered volatile organic content ("VOC") emissions which are preferably reduced or eliminated. Therefore, styrenic foam blowing agent systems which eliminate ethylchloride are desirable.
Several disclosures of use of FC-142b in styrenic foam production have been made. Canadian Patent No. 1,086,450, issued Sep. 23, 1980, discloses production of a styrenic foam having specified characteristics which is produced using a low permeability blowing agent of a specified formula, such as FC-142b. The blowing agent is also disclosed as including a mixture of the low permeability blowing agent with at least one of fluorochloromethane, methylchloride, ethylchloride, chlorodifluoromethane, or 1,2-difluoroethane.
Suh, U.S. Pat. No. 4,636,527, issued January, 1987, discloses a process for the preparation of an alkenyl aromatic foam, such as a polystyrene foam, using a blowing agent mixture comprising about 3 to 45 wt. % carbon dioxide, about 5 to 97 wt. % ethylchloride and from about 0 to 90 wt. % of a fluorocarbon member which is dichlorodifluoromethane, 1-chloro-1,1-difluoroethane or a mixture of the two chlorofluorocarbons.
Park U.S. Pat. No. 4,528,300 discloses a process for producing a polyolefin foam employing a blowing agent comprising 50 to 95 wt. % of FC-142b plus 5 to 50 wt. % of an aliphatic hydrocarbon and/or a halogenated hydrocarbon having a boiling point from 0.degree. to 50.degree. C. Specifically disclosed examples of the aliphatic hydrocarbon and/or halogenated hydrocarbon are ethylchloride and butane. The method disclosed in Park also requires the presence of a stability control agent such as a stearamide in the extrusion mass comprising the polyolefin resin and the blowing agent mixture to produce a stable polyolefin foam. Park does not disclose use of his blowing agent to produce styrenic foams.
Park U.S. Pat. No. 4,640,933 also discloses a process for producing a polyolefin foam which employs a blowing agent mixture comprising, for example, isobutane plus FC-142b, or isobutane plus ethylchloride, along with the stability control agent disclosed in U.S. Pat. No. 4,528,300. Park also does not disclose use of his blowing agent to produce styrenic foams.
Dill, U.S. Pat. No. 3,640,916 discloses a mixture of butane and FC-142b for use as a propellant. Dill is not directed to the use of such a mixture as a styrenic foam blowing agent.
Nakamura, U.S. Pat. No. 3,960,792 discloses a method for producing a styrenic foam using a blowing agent mixture having a specific diffusion rate through the styrenic resin. Nakamura lists FC-142b as a possible blowing agent in his method. Nakamura also discloses that aliphatic hydrocarbons can be used in his method.
Suh, U.S. Pat. No. 4,916,166, issued Apr. 10, 1990, discloses alkenyl aromatic foam bodies produced with FC-142b as the blowing agent. The blowing agent systems disclosed therein are those having greater than 70 wt. % FC-142b. This Suh patent also discloses blends of carbon dioxide, C.sub.1 -C.sub.4 hydrocarbons and greater than 70 wt. % FC-142b as the blowing agent system. There is no disclosure in Suh as to elimination of ethylchloride from FC-142b based blowing agents having less than 70 wt. % FC-142b.
Suh, U.S. Pat. No. 5,011,866, issued Apr. 30, 1991, discloses alkenyl aromatic foam bodies produced using at least 70 wt. % of 1,1,1-trifluoroethane (hereinafter referred to as "FC-143a") or 1,1,1,2-tetrafluoroethane (hereinafter referred to as "FC-134a") as a blowing agent. The blowing agent systems disclosed in this Suh patent also include incorporation of C.sub.1 -C.sub.4 alkanes and carbon dioxide. Like Suh '166, this Suh patent does not disclose elimination of ethylchloride from halogenated ethane blowing agents wherein the halogenated ethane concentration is less than 70 wt. %.
Published European Patent Application 0450513A1, published Oct. 9, 1991, describes production of styrenic foams using a blowing agent and a transient foam control agent described as C.sub.2-30 aliphatic and aromatic carboxylic acids and ester derivatives thereof, C.sub.2-30 aliphatic and aromatic carboxylic acid amides, C.sub.2-30 aromatic or aliphatic sulfones and amide derivatives thereof, C.sub.2-30 aliphatic and aromatic polyhydroxy compounds, C.sub.2-30 aliphatic and aromatic carbamate and carbamate esters, inertly substituted derivatives of these compounds and mixtures thereof. The transient foam control agent is described as useful with numerous blowing agents, including FC-142b and FC-134a. The preferred blowing agent for use with the transient foam control agent is disclosed as a mixture of CO.sub.2 and FC-142b in FC-142b/CO.sub.2 weight ratios from 5/95 to 50/50. The only disclosed examples tested high FC-142b concentration weight ratios of FC-142b/CO.sub.2 of 12.8/1.5 and 10.9/1.5. This disclosure does not suggest use of a FC-142b, a C.sub.4 - C.sub.5 alkane and CO.sub.2 blowing agent system. Nor does it address extrusion mass viscosity control when using such a system.
It is desirable commercially to use styrenic foam blowing agent systems which minimize the amount of the hydrofluorocarbon present due to its expense. Further, it is Applicants' belief that use of amounts of the hydrofluorocarbon in excess of 70 wt. % of the total blowing agent do not achieve any additional benefit in terms of insulating value of the styrenic foams. The prior art disclosures of use of FC-142b, FC-134a and FC-143a, have focused upon use of high concentrations of these blowing agents greater than 70 wt. % or upon their use in conjunction with fluorocarbons of high ozone depletion potential, i.e. Freon-12, or ethylchloride. None of the prior art styrenic foam production disclosures have focused on the need to produce insulating styrenic foams using less than 70 wt. % of a low or zero ozone reactivity potential fluorocarbon, while eliminating ethylchloride use. None of the prior art styrenic foam processes have disclosed a blowing agent system of less than 70 wt. % of one or more of a certain group of halogenated ethanes, such as FC-142b, with a C.sub.4 -C.sub.5 alkane and CO.sub.2, combined with sufficient added plasticizing materials to achieve suitable foam production rates.
It is the general object of the invention to provide a styrenic foam blowing agent system which does not contain ethylchloride. It is an object of the invention to provide a styrenic foam blowing agent having a reduced or zero ozone reactivity potential, which comprises a halogenated ethane of low or zero ozone reactivity potential. It is yet another object to produce a dimensionally stable insulating styrenic foam having an R-value above about 5.0 using the new blowing agent. It is also an object to provide an improved insulating styrenic foam extrusion process. Other objects will be apparent from the specification.
I have found that the objects of the invention can be attained by the use of a blowing agent composition comprising: less than 70.0 wt. %, on a total blowing agent basis, of at least one halogenated ethane selected from the group consisting of 1-chloro-1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, and 1,1,1-trifluoroethane, a C.sub.4 or C.sub.5 alkane, and CO.sub.2, combined with use of sufficient added plasticizing material to achieve the melt viscosity suitable for commercial foam production rates. The method of the invention is capable of producing acceptable foams from polystyrene at suitable production rates. Other benefits of the invention will be addressed below.